1. Field of Invention
This invention relates to induced current linear and rotary position transducers. In particular, this invention is directed to rotary and linear induced current position transducers with improved winding configurations to increase the proportion of the useful output signal component, i.e. those related to transducer position, relative to extraneous ("offset") components of the output signal which are unrelated to transducer position.
2. Description of Related Art
U.S. patent application Ser. No. 08/441,769, filed May 16, 1995, and incorporated herein in its entirety, discloses an incremental induced current position transducer. U.S. patent application Ser. No. 08/645,483, filed May 13, 1996, and incorporated herein in its entirety, discloses an electronic caliper using an induced current position transducer. Both applications disclose associated signal processing techniques for induced current position transducers.
The operation of the induced current position transducers of these applications is generally shown in FIGS. 1 and 2. As shown in FIG. 1, the induced current position transducer 100 comprises a read head 120 that is movable relative to a scale 110. The scale 110 and the read head 120 are preferably formed on printed circuit boards using standard printed circuit board technology.
A plurality of magnetic flux modulators 112 are distributed along a measuring axis 114 of the induced current position transducer 100 at a pitch equal to a wavelength .lambda., which is described in more detail below. The flux modulators 112 have a nominal width along the measuring axis 114 of .lambda./2. The flux modulators 112 have a width d in a direction perpendicular to the measuring axis 114.
The read head 120 includes a generally rectangular transmitter winding 122 that is connected to a drive signal generator 150. The drive signal generator 150 provides a time-varying drive signal to the transmitter winding 122. The time-varying drive signal is preferably a high frequency sinusoidal signal, a pulse signal, or an exponentially decaying sinusoidal signal. When the time-varying drive signal is applied to the transmitter winding 122, the time-varying current flowing in the transmitter winding 122 generates a corresponding time-varying, or changing, magnetic field. Because the transmitter winding 122 is generally rectangularly shaped, the generated magnetic field is substantially constant within a flux region in the central portion of the transmitter winding 122.
The read head 120 further includes a first receiver winding 124 and a second receiver winding 126 positioned on the read head within the flux region inside the transmitter winding 122. Each of the first receiver winding 124 and the second receiver winding 126 is formed by a plurality of first loop segments 128 and second loop segments 129. The first loop segments 128 are formed on a first surface of a layer of the printed circuit board. The second loop segments 129 are formed on another surface of the layer of the printed circuit board. The layer of the printed circuit board acts as an electrical insulation layer between the first loop segments 128 and the second loop segments 129. Each end of each of the first loop segments 128 is connected to one end of one of the second loop segments 129 through feed-throughs 130 formed in the layer of the printed circuit board.
The first and second loop segments 128 and 129 are preferably sinusoidally shaped. Accordingly, as shown in FIG. 1, the first and second loop segments 128 and 129 forming each of the receiver windings 124 and 126 form a sinusoidally-shaped periodic pattern having the wavelength .lambda.. Each of the receiver windings 124 and 126 are thus formed having a plurality of loops 132 and 134.
The loops 132 and 134 in each of the first and second receiver windings 124 and 126 have a width along the measuring axis 114 equal to .lambda./2. Thus, each pair of adjacent loops 132 and 134 has a width equal to .lambda.. Furthermore, the first and second loop segments 128 and 129 go through a full sinusoidal cycle in each pair of adjacent loops 132 and 134. Thus, .lambda. corresponds to the sinusoidal wavelength of the first and second receiver windings 124 and 126. Furthermore, the receiver winding 126 is offset by .lambda./4 from the first receiver winding 124 along the measuring axis 114. That is, the first and second receiver windings 124 and 126 are in quadrature.
The changing drive signal from the drive signal generator 150 is applied to the transmitter winding 122 such that current flows in the transmitter winding 122 from a first terminal 122a, through the transmitter winding 122 and out through a second terminal 122b. Thus, the magnetic field generated by the transmitter winding 122 descends into the plane of FIG. 1 within the transmitter winding 122 and rises up out of the plane of FIG. 1 outside the transmitter winding 122. Accordingly, the changing magnetic field within the transmitter winding 122 generates an induced electromagnetic force (EMF) in each of the loops 132 and 134 formed in the receiver windings 124 and 126.
The loops 132 and 134 have opposite winding directions. Thus, the EMF induced in the loops 132 has a polarity that is opposite to the polarity of the EMF induced in the loops 134. The loops 132 and 134 enclose the same size areas and thus nominally the same amount of magnetic flux. Therefore, the absolute magnitude of the EMF generated in each of the loops 132 and 134 is nominally the same.
There are preferably equal numbers of loops 132 and 134 in each of the first and second receiver windings 124 and 126. Ideally, the positive polarity EMF induced in the loops 132 is exactly offset by the negative polarity EMF induced in the loops 134. Accordingly, the net nominal EMF on each of the first and second receiver windings 124 and 126 is zero and it is intended that no signal is output from the first and second receiver windings 124 and 126 as a result solely of the direct coupling from the transmitter winding 122 to the receiver windings 124 and 26.
When the read head 120 is placed in proximity to the scale 110, the changing magnetic flux generated by the transmitter winding 122 also passes through the flux modulators 112. The flux modulators 112 modulate the changing magnetic flux and can be either flux enhancers or flux disrupters.
When the flux modulators 112 are provided as flux disrupters, the flux modulators 112 are formed as conductive plates or thin conductive films on the scale 110. As the changing magnetic flux passes through the conductive plates or thin films, eddy currents are generated in the conductive plates or thin films. These eddy currents in turn generate magnetic fields having a field direction that is opposite to that of the magnetic field generated by the transmitter winding 122. Thus, in areas proximate to each of the flux disrupter-type flux modulators 112, the net magnetic flux is less than the net magnetic flux in areas distant from the flux disrupter-type flux modulators 122.
When the scale 110 is positioned relative to the read head 120 such that the flux disrupters 112 are aligned with the positive polarity loops 132 of the receiver winding 124, the net EMF generated in the positive polarity loops 132 is reduced compared to the net EMF generated in the negative polarity loops 134. Thus, the receiver winding 124 becomes unbalanced and has a net negative signal across its output terminals 124a and 124b.
Similarly, when the flux disrupters 112 are aligned with the negative polarity loops 134, the net magnetic flux through the negative polarity loops 134 is disrupted or reduced. Thus, the net EMF generated in the negative polarity loops 134 is reduced relative to the net EMF generated in the positive polarity loops 132. Thus, the first receiver winding 124 has a net positive signal across its output terminals 124a and 124b.
When the flux modulators 112 are provided as flux enhancers, this result is exactly reversed. The flux-enhancer-type flux modulators 112 are formed by portions of high magnetic permeability material provided in or on the scale 110. The magnetic flux generated by the transmitter winding 122 preferentially passes through the high magnetic permeability flux-enhancer-type flux modulators 112. That is, the density of the magnetic flux within the flux enhancers 112 is enhanced, while the flux density in areas outside the flux enhancers 112 is reduced.
Thus, when the flux enhancers 112 are aligned with the positive polarity loops 132 of the second receiver winding 126, the flux density through the positive polarity loops 132 is greater than the flux density passing through the negative polarity loops 134. Thus, the net EMF generated in the positive polarity loops 132 increases, while the net EMF induced in the negative polarity loops 134 decreases. This appears as a positive signal across the terminals 126a and 126b of the second receiver winding 126.
When the flux enhancers are aligned with the negative polarity loops 134, the negative polarity loops 134 generate an enhanced EMF relative to the EMF induced in the positive polarity loops 132. Thus, a negative signal appears across the terminals 126a and 126b of the second receiver winding 126. It should also be appreciated that, as outlined in the incorporated references, both the flux enhancing and flux disrupting effects can be combined in a single scale, where the flux enhancers and the flux disrupters are interleaved along the length of the scale 110. This would act to enhance the modulation of the induced EMF, because the effects of both types of flux modulator additively combine.
As indicated above, the width and height of the flux modulators 112 are nominally .lambda./2 and d, respectively, while the pitch of the flux modulators 122 is nominally .lambda.. Similarly, the wavelength of the periodic pattern formed in the first and second receiver windings 122 and 124 is nominally .lambda. and the height of the loops 132 and 134 is nominally d. Furthermore, each of the loops 132 and 134 encloses a nominally constant area.
FIG. 2A shows the position-dependent output from the positive polarity loops 132 as the flux modulators 112 move relative to the positive polarity loops 132. Assuming the flux modulators 112 are flux disrupters, the minimum signal amplitude corresponds to those positions where the flux disrupters 112 exactly align with the positive polarity loops 132, while the maximum amplitude positions correspond to the flux disrupters 112 being aligned with the negative polarity loops 134.
FIG. 2B shows the signal output from each of the negative polarity loops 134. As with the signal shown in FIG. 2A, the minimum signal amplitude corresponds to those positions where the flux disrupters 112 exactly align with the positive polarity loops 132, while the maximum signal output corresponds to those positions where the flux disrupters exactly align with the negative polarity loops 134. It should be appreciated that if flux enhancers were used in place of flux disrupters, the minimum signal amplitudes in FIGS. 2A and 2B would correspond to the flux enhancers 112 aligning with the negative polarity loops 134, while the maximum signal amplitude would correspond to the flux enhancers 112 aligning with the positive polarity loops 132.
FIG. 2C shows the net signal output from either of the first and second receiver windings 124 and 126. This net signal is equal to the sum of the signals output from the positive and negative polarity loops 132 and 134, i.e., the sum of the signals shown in FIGS. 2A and 2B. The net signal shown in FIG. 2C should ideally be symmetrical around zero, that is, the positive and negative polarity loops 132 and 134 should be exactly balanced to produce a symmetrical output with zero offset.
However, a "DC" (position independent)component often appears in the net signal in a real device. This DC component is the offset signal V.sub.o. This offset V.sub.o is an extraneous signal component which complicates signal processing and leads to undesirable position measurement errors. This offset has two major sources.
First, the full amplitude of the transmitter field passes through the first and second receiver windings 124 and 126. As outlined above, this induces a voltage in each loop 132 and 134. The induced voltage is nominally canceled because the loops 132 and 134 have opposite winding directions. However, to perfectly cancel the induced voltage in the receiver windings requires the positive and negative loops 132 and 134 to be perfectly positioned and shaped, for a perfectly balanced result. The tolerances on the balance are critical because the voltages induced directly into the receiver winding loops 132 and 134 by the transmitter winding 122 are much stronger than the modulation in the induced voltage caused by the flux modulators 112. In practice, fabrication tolerances always prevent perfect balance.
Second, the spatially modulated field created by the flux modulators 112 also exhibits an average position-independent offset component. That is, the flux modulators 112 within the magnetic field generated by the transmitter winding 122 all create the same polarity spatial modulation in the magnetic field. For example, when flux disrupters are used, the induced eddy current field from the flux modulators has an offset because the flux disrupters within the transmitter field all create a same polarity secondary magnetic field. At the same time, the space between the flux disrupters does not create a secondary magnitude field.
Thus, each positive polarity loop 132 and each negative polarity loop 134 of the receiver windings 124 and 126 sees a net magnetic field that varies between a minimum value and a maximum value having the same polarity. The mean value of this function is not balanced around zero, i.e., it has a large nominal offset. Similarly, when flux enhancers are used, the field modulation due to the flux enhancers has a bias because the enhancers within the transmitter winding 122 all create the same field modulation, while the space between the modulators provides no modulation. Each positive and negative polarity loop 132 or 134 of each receiver winding 124 or 126 therefore sees a spatially modulated field that varies between a minimum value and a maximum value having the same polarity. The mean value of this function also has a large nominal offset.
A receiver winding having an equal number of similar positive and negative polarity loops 132 and 134 helps eliminate the offset components. However, any imperfection in the balance between the positive and negative polarity loops 132 and 134 allows residual offsets according to the previous description.
Both of these offset components are expected to be canceled solely by the symmetry between the positive and negative polarity loops 132 and 134 in the first and second receiver windings 124 and 126. This puts very stringent requirements on the manufacturing precision of the receiver windings 124 and 126. Experience in manufacturing the transducer 100 indicates it is practically impossible to eliminate this source of error from the induced current position transducer 100.
Any signal component which is independent of the transducer position, such as the aforementioned offset components, is regarded as an extraneous signal to the operation of the transducer. Such extraneous signals complicate the required signal processing circuitry and otherwise lead to errors which compromise the accuracy of the transducer.
Other related art has disclosed simple winding configurations with the extraneous coupling between the transmitter and receiver windings reduced simply by placing the receiver winding distant from the field produced by the transmitter winding. However, the effectiveness of this technique alone depends on the degree of separation between the transmitter and receiver windings, and hence this technique contradicts the need for high accuracy position sensors of compact size. Alternatively, the transmitter field can be confined with magnetically permeability materials so that the effectiveness of a given degree of separation is increased. However, this technique leads to additional complexity, cost, and sensitivity to external fields, in a practical device.
Furthermore, the simple winding configurations disclosed in association with these techniques include no means for creating a device with a measuring range significantly exceeding the span of the transmitter and receiver winding. In addition, the simple winding configurations provide no means for significantly enhancing the degree of output signal change per unit of displacement for a given measuring range. Thus, the practical measuring resolution of these devices is limited for a given measuring range.
The need for a high accuracy inductive measuring device which rejects both extraneous signal components and external fields, is compact, of simple construction, and capable of high resolution measurement over an extended measuring range without requiring increased fabrication and circuit accuracies, has therefore not been met previously.